How can I calculate power consumption of my PC in Watt?

The definitive way would be to buy a Kill A Watt and plug your computer into it. That way you'll be measuring actual power usage instead of trying to estimate.

• Amazon link
• ThinkGeek link
• Jeff Atwood's "review"

You don't need to worry about all the internal bits of the machine to calculate worst case power usage. The wattage listed on the power supply is the maximum the machine can draw when under heavy load. It will typically draw far less. If you need a more accurate number then the Kill-a-Watt solution is recommended.

Remember that electricity is usually sold in units of kilowatt-hours. 1 kWh is the energy used by a 1000-watt widget operating for 1 hour.

So if your power supply is rated at 500 watts, and you leave it on for 24 hours, at the end of that time you've used, at most, 500 x 24 = 12000 watt-hours or 12 kWh.

Actual usage is probably far less, but without a meter it's very difficult to calculate.

I know an answer has already been accepted, but I just saw this:

Microsoft Research Releases "Joulemeter" Power Consumption Estimator for Windows

Joulemeter is a software based mechanism to measure the energy usage of virtual machines (VMs), servers, desktops, laptops, and even individual softwares running on a computer."

Joulemeter estimates the energy usage of a VM, computer, or software by measuring the hardware resources (CPU, disk, memory, screen etc) being used and converting the resource usage to actual power usage based on automatically learned realistic power models."

Download --> Joulemeter

To add to this: ESSaver is a free tool that shows the energyconsumption of the computer and even per process.